Organizing IdeaEnergy — Understandings of the physical world are deepened by investigating matter and energy.
Guiding QuestionIn what ways can interactions lead to physical change?
Learning OutcomeStudents analyze forces and relate them to interactions between objects.
This lesson covers: Newton’s Third Law: for every action force there is an equal and opposite reaction force; how one object experiences an action force while another experiences a reaction force in an interaction; demonstrating and representing action-reaction force pairs in various situations. (Newton’s First and Second Laws are introduced here as supporting context and extend into later grades.)
1
Newton's three laws
In 1687, Isaac Newton published three laws of motion. They
describe how forces cause objects to start moving, change speed, or change
direction, and they apply to everything from a falling apple to a spacecraft.
law 1
Inertia
An object's motion does not change unless an unbalanced force acts on it.
law 2
F = m × a
The acceleration of an object equals the force on it divided by its mass.
law 3
Action–Reaction
For every action, there is an equal and opposite reaction.
a note on the third law
The third law is the least intuitive of the three. It explains how rockets,
walking, and swimming all work, so it gets a longer section below.
2
First Law — Inertia
An object at rest stays at rest, and an object in motion stays in
motion at constant speed and direction, unless acted on by an unbalanced
external force. This property of matter — the resistance to changes
in motion — is called inertia.
why don't real objects move forever?
On most surfaces, friction opposes motion and slowly brings
moving objects to rest. The puck above slides far because the ice has very
little friction. In deep space, with no surface or air to slow it, the same
puck would continue at constant velocity indefinitely.
everyday inertia
Seatbelts work because of inertia. When a car stops suddenly, a passenger's
body continues moving forward at the car's previous speed. The seatbelt
provides the force that decelerates the passenger along with the car.
3
Second Law — Force = Mass × Acceleration
The acceleration of an object depends on two things: the size of the force
applied to it, and the object's mass. The relationship is:
F = m × a
A larger force produces more acceleration. A larger mass produces less
acceleration for the same force. Adjust the sliders below to see how
changing each one affects the cart.
Force5
Mass3
F = 5, m = 3 → a = 1.67
a familiar example
Push a shopping cart and a parked car with the same force. The cart accelerates
much more than the car because it has far less mass. Same force, different
masses, different accelerations.
4
Third Law — Action and Reaction
For every action, there is an equal and opposite reaction.
Whenever object A exerts a force on object B, object B exerts a force on A
that is equal in size and opposite in direction. The two forces together are
called an action–reaction pair.
Blue skater mass3
Red skater mass3
Adjust the masses, then push to see how mass affects each skater's acceleration.
the most common mistake
A common misconception is that the heavier skater experiences a smaller force
from the push. By Law 3, both skaters experience the same size
force. What differs is their acceleration: by Law 2,
the heavier skater accelerates less and the lighter skater accelerates more.
a key detail
Action and reaction act on two different objects, so they do
not cancel each other out. The force Blue exerts on Red accelerates Red. The
force Red exerts on Blue accelerates Blue. Two forces, two objects, two
separate accelerations.
5
Examples of the third law
Action–reaction pairs appear in many everyday motions. In each example below,
the red arrow shows the action and the blue
arrow shows the reaction.
Walking
Your foot pushes the ground backward; the ground pushes
you forward. Without the reaction force, your foot would
slip in place.
Burning fuel ejects gas downward at high speed. The gas
exerts an equal upward force on the rocket. No ground
is needed, so rockets work in the vacuum of space.
action: rocket → gas (down)
reaction: gas → rocket (up)
Swimming
A swimmer's hand pushes water backward; the water pushes
the swimmer forward. The same principle as walking, with
water replacing solid ground.
action: hand → water (back)
reaction: water → swimmer (forward)
Bouncing ball
The ball pushes down on the ground; the ground pushes
up on the ball. The upward reaction force is what sends
the ball back into the air.
To find an action–reaction pair, ask what the object is pushing on. That push
is the action; the equal force back is the reaction. A jumping frog pushes
down on the lily pad, and the lily pad pushes up on the frog.
6
How the three laws connect
The three laws work together to describe an object's motion.
Law 1 establishes that motion does not change without a force.
Law 2 quantifies how much a force changes motion: a = F ÷ m.
Law 3 establishes that every force comes in a pair acting on
two different objects.
Together, these laws describe the same forces from earlier lessons — friction,
gravity, spring forces, and the attractions between particles.
summary
Law 1: Motion remains constant unless an unbalanced force
acts on it. Law 2: a = F ÷ m. Law 3: Every force has an equal and opposite reaction force.